In recent years, with rapid development of power supply technology, a switching mode stabilized power supply is trending towards miniaturization, high frequency and integration, and the high-efficiency switching power supply has been widely applied. A flyback-mode power supply circuit is one of the low-power switching power supply particularly suitable for, such as, household appliances, battery chargers and many other designs, due to its a number of advantages, e.g., simplicity of circuit, capability of efficiently providing direct-current output and the like.
A flyback switching power supply refers to a switching power supply with a flyback high-frequency transformer isolating input and output circuits. The term “flyback” specifically refers to that inductors connected in series in an output circuit are in a discharging state when the input is in high level (a switching diode is turned on) and are in a charging state when the input is low (the switching diode is turned off). FIG. 1 shows a schematic diagram of a flyback switching power supply circuit used in a liquid crystal display in the prior art. As shown in FIG. 1, the switching power supply circuit mainly includes a voltage input end, a control IC, a power MOS transistor, a transformer, a rectifier diode and an output capacitor.
Specifically, the MOS transistor is controlled by the control IC, and is turned on or off through a pulse width modulation signal generated by the control IC. When the power MOS transistor is turned on, the inductive current in a primary winding of the transformer starts rising. At this moment, the rectifier diode is turned off due to a secondary winding, and thus energy is stored in the transformer. When the power MOS transistor is turned off, the inductive voltage of the primary winding of the transformer is inverted. At this moment, the rectifier diode is turned on, and the energy in the transformer is supplied to a load through the rectifier diode.
However, in the topological circuit design of the above-mentioned flyback switching power supply circuit, the control IC is used to directly control turn-on and turn-off of the MOS transistor. Due to the parasitic capacitance effect in the transformer, after the MOS transistor is turned off, the drain (D) of the MOS transistor may not be immediately stabilized but tend to be stabilized in manner of damped vibration (as shown in FIG. 2). Since effect from damped vibration is not considered in this process, high switching loss of the MOS transistor is caused.
Accordingly, one of problems dedicated in the field is how to solve the above-mentioned problems, such that the switching loss of the MOS transistor in the flyback switching power supply is reduced.